The use of safety circuits to prevent overloading an extensible boom crane and thus damaging the boom or causing the device to topple are well known in the art, but of the multitude of devices, all incorporate compromises which result in significant shortcomings in the applied systems.
The prior art safety devices may be grouped in two broad categories, one category which comprises those systems which prevent an operator from extending or lowering a boom into a region of unsafe operation and a second type which provides an alarm to warn an operator of an impending disaster such as crushing the boom or tipping the crane.
R. Sterner, U.S. Pat. No. 3,641,551 on "Safe Load Control System For Telescopic Crane Booms" issued Feb. 8, 1972 is typical of the first type of safety systems which include both an overload prevention and an indicator system. These devices typically include an electrical circuit responsive to a first electrical network which changes as a function of boom length and a second electrical circuit which changes as a function of boom angle for providing a warning indication and inhibiting the operation of a hydraulic boom drive means.
The safety control systems such as found in Sterner include series circuits comprised of a large number of electrical contacts that are subject to contamination and failure. A malfunction of the system can lead to an inoperative boom in one failure mode or a short circuited, bypassed safety system in a second failure mode which would allow an operator unknowingly to exceed safe limits of his device.
C. Kezer et al, in U.S. Pat. No. 3,740,534 on "Warning System For Load Handling Equipment" issued June 19, 1973 is exemplary of systems adapted primarily to provide an operator with a warning that continued operation will be hazardous and may result in a catastrophic failure. Systems such as this do not provide a system inhibit function such as provided by the Sterner type systems discussed above and thus do not provide the safety feature of a system capable of overriding the actions of an imprudent operator. Kezer et al illustrates the current trend of replacing electro-mechanical control systems with electronic logic systems. However, the advances in this phase of the art have been relatively complex and incorporate extensive electronic systems which are costly to manufacture and subject to a high failure rate due to the large number of interdependent circuits.